1. Field
The invention relates to a receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device with such a receiving part. The head of the bone anchoring element is locked in the receiving part by compression of a head receiving portion of the receiving part laterally surrounding the head by means of a locking ring. The head receiving portion of the receiving part has an exterior surface with a tapered portion and the locking ring has an interior surface with a curved portion which presses against the tapered portion of the exterior surface of the head receiving portion to compress the head receiving portion to clamp the head. The bone anchoring device can be realized, for example, in the form of a polyaxial bone screw allowing a pivotal movement of the head.
2. Description of Related Art
Various designs of polyaxial bone screws are known wherein each design has particular characteristics and wherein different locking principles are used.
U.S. Pat. No. 5,443,467 describes a polyaxial bone screw having a screw element and a receiving part and a pressure element acting onto the head of the screw element to press it down against the seat in the receiving part in order to lock the rotational position of the head. A bone screw usually is preassembled so that the screw element is screwed into the bone with a receiving part mounted on the screw element.
Other bone screws are known wherein the head is clamped from the side to lock the rotational position.
U.S. Pat. No. 4,946,458 describes a bone screw with a receiving part consisting of two spherically-shaped halves pressing from two lateral sides onto the spherical head. The halves are held together in their lower portion by a ring.
U.S. Pat. No. 5,672,179 describes a bone screw with a receiving part with a conically shaped seat and a conically shaped pressure element which exerts pressure onto the head from above and from the side. If the cone angle has a value lying within a specific range self-locking of the pressure element within the receiving part takes place which allows to preliminary lock the head within the receiving part while the rod is still movable in order to allow the adjustment of its position.
U.S. Pat. No. 6,063,090 A describes a bone screw with a receiving part having a conically-shaped cavity accommodating the head wherein a spring chuck is provided in the cavity which is downwardly and radially compressible to clamp the head. With this bone screw it is possible to click the receiving part onto the head of the screw element which allows to screw the screw element into the bone and to connect it to the receiving part thereafter.
U.S. Pat. No. 5,728,098 describes a bone screw for connection to a spinal rod comprising a screw element and a receiver member which has slits provided at the bottom of the rod receiving channel and wherein two ring shaped compression members made of a shape-memory alloy are provided at the lower side and the upper side of the receiver member, respectively. The compression members contract about the portions of the receiver member when the temperature is elevated so that the rod is clamped in the channel.
U.S. Pat. No. 5,549,608 describes a polyaxial bone screw with a screw element with a spherical head and a coupling element to couple the screw element into a spinal rod. The coupling element has a tapered lower portion including a slotted interior chamber in which the spherical head is initially polyaxially disposed. The coupling element further comprises a recess for receiving the head. In addition, a locking ring surrounding the lower portion of the coupling element and a cylindrical rod securing sleeve which fits over the coupling element are provided. A top locking nut is used to exert pressure onto the rod securing sleeve. The head is locked in the interior chamber by means of the locking ring which is pressed down by the rod securing sleeve.
U.S. Pat. No. 5,733,285 describes a similar polyaxial bone screw wherein in one embodiment the rod securing sleeve is omitted and the rod directly presses onto a locking collar disposed around the tapered and colleted portion of the coupling element. The locking collar has to be placed onto the coupling element from above. It is not secured against escaping towards the upper end and against rotation when the rod is not inserted. Furthermore, the size of the known bone anchoring device is quite large as the locking collar and the top locking nut extend substantially outward from the diameter of the coupling element.
WO 2007/038350 A2 discloses an apparatus for connecting a bone anchor to a support rod including a connector body and a cap. The connector body has a socket for insertion, angulation and removal a bone anchor, the socket having a section with a spherical outer surface. A sleeve is provided which is configured to fit over the connector body, the sleeve having a conical inside wall which is tangential to the spherical outer surface of the spherical section. The contact between the spherical section and the conical inside wall occurs around an annulus defining a circular contact zone. This circular contact zone provides uniform compression of the chamber receiving the bone anchor by the sleeve. The sleeve extends over the whole length of the socket.
US 2005/0080415 A1 describes a polyaxial bone anchor for attaching a rod to a bone comprising an anchor member and a body member having a U-shaped channel for receiving the rod and a compressible recess for receiving a head of the anchor member. A portion of an exterior surface of the compressible recess is tapered and a collar is slidably disposed about the body member. The collar comprises an interior surface portion which is tapered and which cooperates with the tapered exterior surface portion of the compressible recess.